Amazon.com Inc. finally revealed financial details about its secretive cloud computing division, but it was otherwise business as usual in the first quarter for the e-commerce giant.

The Seattle company reported a loss despite rapidly rising sales, as it continued to spend heavily to fund a variety of projects such as drone delivery and streaming-video deals, as well as warehouse construction. And while some investors fret that Amazon is spreading itself thin, such as with the unpopular Fire smartphone, the company reaffirmed...